It is desired for wireless communication systems to allow as many signals as possible to be transmitted at the same frequency from the viewpoint of efficient use of limited frequency resources. Many researches and development have been made of interference cancellers for this purpose. In fact, interference cancellation is attracting attention as a technique that can increase system capacity.
FIG. 1 is a block diagram of a conventional interference canceller 300, which is generally installed in a receiver in a wireless communications system. This type of interference canceller is called a replica generation type interference canceller because it generates a replica of the received signal and separates a desired wave from other signals (interference waves).
In operation of the conventional replica generation type interference canceller, the channel estimator 318 estimates the level fluctuation and phase rotation in the channels of the desired wave and the interference wave. The coefficient-variable filters 314 and 315 generate desired signal replicas and interference signal replicas, respectively, for all possible candidates of symbol sequence. Each of the desired signal replica and the interference signal replica can be generated by calculating convolution of every possible candidate of the symbol sequence and the estimated channel value. The adder 316 adds one of the desired signal replicas to one of the interference signal replicas to produce a set of received signal replicas. The maximum likelihood sequence estimator 320 determines a pair of symbol sequence candidates of the desired signal and the interference signal that provide the received signal replica closest to the actually received signal. The maximum likelihood sequence estimator 320 outputs the symbol sequence candidate of the desired wave as the determination result, thereby removing the interference.
By removing the interference signal from the received signal in an adaptive manner, multiple signals can be transmitted at the same time at the same frequency, and the frequency utilization efficiency is improved.
An interference canceller used in a multi-rate transmission system is also proposed in, for example, Japanese Patent Laid-Open Publication No. 11-251959A. In this publication, signals are received at an array antenna in a DS-CDMA system using both a high-speed channel with a high transmission rate and a low-speed channel with a low transmission rate. An array antenna interference replica generation unit is provided to generate an interference replica of the high-speed channel for the purposes of removing the interference due to the high-speed channel from the received signal, and of improving the transmission quality of the low-speed channel.
The conventional interference canceller of the replica generation type can remove interference by generating replicas of the desired signal and the interference signal. However, since the conventional technique is based on the assumption that the band widths of the desired wave and the interference wave are the same, sufficient effect of interference cancellation cannot be achieved if a desired wave requiring a wide band and an interference wave expecting a narrow band are combined to form the received signal. To maintain the frequency utilization efficiency of the interference canceller of replica generation type, combinations of a wide-band signal and a narrowband signal have to be avoided at the same frequency.